Several high strength proppants are available on the market. While these proppants have superior crush strength relative to frac sand, they are considerably more expensive so the question presented to the producer is one of pure economics balancing proppant failure against cost and productivity. In some cases, the economics of using the superior proppant may be marginal so alternative methods become attractive. One such alternative is a modification of the proppant schedule to place a regular strength proppant deep in the fracture and a high strength proppant next to the wellbore where closure stresses will be greatest. The term “tail in” has become used to describe the use of a different proppant in the final proppant stage of a multifracture treatment. Therefore a “tail-in” is pumped either to control proppant flowback, maximize conductivity nearest the wellbore, or both.
Fines are a problem in hydraulic fracturing of wells to recover trapped oil and gas. Fines can come from loosely consolidated strata, such as sandstone, whose grains flow into the well along with the oil and gas. The fracturing process itself can create fine pieces of rock and strata that enter liquid flow streams towards the wellbore. The substantial pressures and harsh environment within the field can also cause proppants to fail and generate fines, particularly during cyclic shut-in periods. The high stress exerted on the proppant pack can also lead to the embedment of the proppant pack into the faces of the created fractures. This embedment process will itself creates fines that can enter the proppant pack and be transported toward the wellbore. Portions of these fine materials are sufficiently small that they can become entrained in the water, oil and gas streams that move under pressure towards the relatively lower pressure wellbore. These fine materials will increase in concentration as they move along the fracture toward the wellbore and can ultimately clog desirable pore openings and channels to the detriment of the well's conductivity. Gravel packs and screens have been used around the wellbore to help protect, among other things, against loss of conductivity from fines movement.
Others have addressed the issue of fines generation and protecting the propped field against loss of conductivity and/or permeability that can occur with the movement of the fines during production.
WO 2012/085646 includes a detailed background discussion of hydraulic fracturing terminology and techniques and specifically teaches the sequential use of a fine proppant followed by a re-opening of the fractures and introduction of a larger proppant. However in this approach a smaller sized proppant is to be placed along the fracture faces specifically to minimize the entry of formation fines into the proppant pack. It is not designed nor can it be expected to address the generation of fines (from proppant crushing) or the control of movement of the fines (that are in the packed fracture) during production of the well. Such a process uses the fine proppant to hold open small cracks so that they can be re-expanded in the re-fracking step to a size sufficient for the larger proppant.
In its 2005 paper “Conductivity Endurance”, Halliburton describes the adverse effects of fines penetration into proppant packs with the attendant reduction in conductivity. On pages 23-24, five mechanisms of particle deposition are presented: (1) surface deposition of particles, (2) pore-throat bridging and accumulation, (3) internal cake formation, (4) external cake formation, and (5) infiltration sedimentation. The solutions proposed in the paper included the use of (a) resin coated sands, (b) mechanical exclusion methods such as mechanical screens or the “frac-packing technique”, and (c) chemical treatments that include (i) a proprietary surface modification agent designed to form a tacky exterior coating, (ii) chemical flocculants, (iii) organic cationic polymers, (iv) inorganic polymers, (v) oil-wetting surfactants, and (vi) clay stabilizing agents.
The 2007 article “Frac Packing: Fracturing For Sand Control”, Middle East & Asia Reservoir Review, Number 8, pp. 36-49 (2007) describes the frac-packing technique for controlling fines as the simultaneous fracturing of the well with the formation of the gravel pack to hold back formation sand behind a pre-positioned screen that holds back the gravel.
A 1992 paper entitled “Fracture Conductivity Loss Due to Proppant Failure and Fines Migration”, CIM 1992 Annual Technical Conference, Calgary, Canada (Jun. 7-10, 1992) reports on the effects of fines migration on a proppant pack containing two different proppants simulating tail-ins. As noted on page 3 of that article, tests were performed to simulate the placement of sand in the front end of a fracture and a higher strength proppant nearest the wellbore, i.e., a tail-in.
The disclosures of the above references and all other references mentioned in this document are hereby incorporated by reference.
It would be desirable to have a method that helped to minimize proppant failure that contributes to the presence of fines and for controlling the movement of fines in the fractured field through the proppant pack and, ultimately, to the wellbore and any screens or gravel pack found there so as to maintain conductivity through the fractured field.
It would also be desirable to have a fines control system that did not require that well operators add expensive, new equipment or systems in order to achieve better fines control and maintained conductivity.